1. Field of the Invention
The present invention relates to an ink jet apparatus for discharging from a nozzle an ink as ink droplets.
2. Discussion of the Background
For ink jet printers, inks such as water-based inks, oil-based inks, solvent inks and UV inks have been employed. From such various inks, a proper ink is selected as needed, depending on the kind of recording media on which an image is printed by the deposition of the ink or kind of a fixing mechanism adopted by an ink jet printer.
Water-based inks and oil-based inks are generally common in that they are used for water absorbing media. They however differ in fixing mechanism. The water-based inks are fixed by evaporation of a solvent therefrom and penetration of the residue into media upon fixation, while the oil-based inks tend to be fixed only by their penetration into media upon fixation. Since they are both fixed by the penetration into media, fixation to non-absorbing media is very difficult for both of them.
Accordingly, solvent inks have been frequently used for non-absorbing media conventionally. The fixing mechanism of solvent inks however mainly depends on the evaporation of the solvent contained therein so that a system for collecting a volatile organic compound generated upon evaporation of the solvent becomes necessary.
In addition, solvent inks exhibit high volatility so that owing to the evaporation and drying of the solvent, clogging occurs frequently at a nozzle formed in an orifice plate and it sometimes disturbs jetting of ink droplets. It is therefore necessary to carry out maintenance such as spitting or purging at frequent intervals.
Under such circumstances, use of UV inks (ultraviolet curable ink) has attracted attention in recent years. Fixation of UV inks occurs by photo-curing reaction. A photo initiator contained in the inks reacts with a reactive monomer or oligomer to cause transformation into the corresponding polymer. By this, UV inks are fixed well to non-absorbing media. The above-described reaction is completed in so short a time that no solvent is generated. Moreover, owing to very low volatility, they rarely cause clogging, which is a problem of solvent inks resulting from evaporation and drying of a solvent at a nozzle formed in an orifice plate. As such excellent fixation mechanism, low volatility and viscosity reduction of inks are appreciated more, there has increasingly been a demand for ink jet recording system using UV inks for non-absorbing media in recent years.
UV inks are however accompanied with the drawback that the fluidity between the ink and an ink repellent layer formed on the peripheral surface of an ink-jetting nozzle, which surface is a part of the surface (ink ejection side) of an orifice plate equipped with the nozzle, deteriorates with the passage of time. This disturbs straight ejection of ink droplets or increases frequency of misfiring. Such a deterioration in the fluidity of UV inks with the passage of time differ, depending on the combination of the ink repellent layer and ink composition so that it is important to optimize the combination of the ink repellent layer and ink composition in order to prevent such a deterioration.
Such a deterioration in fluidity of the UV inks with the passage of time cannot be grasped by the conventional method of measuring a contact angle between each ink and an ink repellent film. This leads to a difficulty in finding of the most suited combination of the ink repellent layer and UV ink. This problems applies not only UV inks but also the other inks such as inks containing a nonaqueous solvent and colorant.
An object of the present invention is to find, in a simple manner, a most suited combination of an ink repellent layer formed on the peripheral surface of a nozzle of an orifice plate and an ink, which combination does not cause a deterioration in the fluidity of the ink between the ink repellent layer and the ink even after the passage of time, and to provide an ink jet apparatus capable of stably discharging ink droplets from the nozzle.
The above-described object of the present invention is attained by the novel ink jet apparatus of the present invention.
According to the novel ink jet apparatus of the present invention, an ink repellent layer similar to that having an ink repellent property satisfying a slide-down speed of 2 mm/sec or greater at 25xc2x0 C. either just after immersion or 100 hours after immersion is formed on at least the peripheral surface of a nozzle which surface constitutes an ink-ejection-side surface of an orifice plate, the speed being determined by a slide-down speed evaluation method for measuring the slide-down speed of ink droplets by immersing a test piece having an ink repellent layer formed on the surface thereof in an ink which is curable by radiation and is to be filled in the pressure chamber of the ink jet apparatus, pulling the test piece out of the ink, fixing the test piece at an angle of 45 degrees with respect to a horizontal plane, and dropping 10 to 20 xcexcl of the ink to the fixed test piece.